1. Field of the Invention
The present invention relates to a process for producing a low water-absorbing artificial lightweight aggregate from coal ash as a main starting material.
2. Description of Prior Art
There has recently been a tendency that structures such as buildings are made more weight-saving and heat-insulating from the viewpoints of energy-saving and resource-saving and the viewpoint of improving the earthquake-proofness. Accordingly, artificial lightweight aggregates are used as the aggregate for lightweight concrete.
These artificial lightweight aggregates must have such characteristics that they are lightweight and have high compressive strength and low water absorption. Further, it is desirable that they can be produced easily and inexpensively.
As such artificial lightweight aggregate, expanded shale aggregate has been conventionally used.
There are advantages that expanded shale artificial lightweight aggregate can be produced very inexpensively and has excellent physical properties. Recently, the quarrying of shale is often limited by reason of environmental protection.
Thus, the development of artificial lightweight aggregate to be substituted for the expanded shale aggregate has been extensively made, and methods using coal ash formed in the combustion of coal, as a main starting material are drawing attention.
Such methods are also drawing attention from the viewpoint of developing the effective utilization of coal ash formed in great quantities with rapid increase in the consumption of coal as an energy source.
Heretofore, various processes for producing artificial lightweight aggregate from coal ash as a main raw material (hereinafter referred to sometimes as coal ash artificial lightweight aggregate) have been proposed. These processes are disclosed, for instance, in Japanese Patent Publication Nos. 36(1961)-12580, 38(1963)-25820, 40(1965)-16270, 41(1966)-8239 and 54(1979)-6468 and Japanese Patent Provisional Publication No. 59(1984)-39757.
However, the production of artificial lightweight aggregate from coal ash as a main starting material in Japan has not been industrially done as yet, mainly because there are disadvantages that in the production of the coal ash artificial lightweight aggregate by the above-mentioned known processes on industrial scale, the rejection rate is high and hence, manufacturing cost is high as compared with expanded shale artificial lightweight aggregate commercially available in Japan.
The present inventors have made studies on the above described aggregates, and as a result, they have found that the known processes are suitable only for use in the production of a small amount of artificial lightweight aggregate on laboratory scale. These known processes have problems in production thereof on industrial scale in that granules (material to be calcined) are broken during manufacturing and hence, there is a difficulty in obtaining artificial lightweight aggregate having the desired shape, and as a result its manufacturing cost is increased.
Accordingly, it is eagerly demanded to provide a novel lightweight aggregate to be substituted for the expanded shale artificial lightweight aggregate and to develop a technique for the effective utilization of coal ash. However, the production of coal ash artificial lightweight aggregate has not been industrialized in Japan as yet.
The present inventors had already invented a process for producing coal ash artificial lightweight aggregate on industrial scale and the invention was now pending as U.S. Patent 4,701,222. The invention relates to a process for producing an artificial lightweight aggregate comprising granulating a starting powdery material containing at least 80 wt.% of coal ash having a Blaine specific surface area of 2,000 to 9,000 cm.sup.2 /g to give granules having an apparent specific gravity of 1.1 to 1.8 and a crushing strength of 0.6 kgf or above at room temperature as well as after heating at 600.degree. C.; and calcining the granules at a temperature of 1,100.degree. to 1,600.degree. C.
The invention relates to a process in which the granules having a crushing strength within a specific range and an apparent specific gravity within a specific range are prepared so that the wear pulverizing rate of the granules is low, that is the granules are hardly broken during the calcining stage and hence, artificial lightweight aggregate can be advantageously produced on industrial scale.
The inventors have made further studies on the above process and found a novel process which allows the characteristics of the granules to be improved. It has also been found that the production of the artificial lightweight aggregate can be much more advantageously carried out on industrial scale by said process.